1. Field of the Invention
The present invention relates to a device hereinafter, referred to as a light emitting device) having an element hereinafter, referred to as a light emitting element) sandwiching a luminous material between electrodes. In particular, the present invention relates to a light emitting device having a light emitting element (hereinafter, referred to as an EL element) using an organic compound from which EL (electro luminescence) is obtained as a luminous material. Note that, an organic EL display and an organic light emitting diode (OLED) are included in a light emitting device of the present invention.
Further, the luminous materials that may be used for the present invention include all the luminous materials that luminesce (phosphorescence and/or fluorescence) via a singlet excitation or a triplet excitation, or via both the excitations.
2. Description of the Related Art
In recent years, the development of an EL element with an organic EL film as a light emitting layer is progressing, and the EL elements using various organic EL films have been proposed. Also, experiments to realize a flat panel display using a light emitting device with such an EL element as a light emitting element have been conducted.
As a light emitting device using an EL element, there are known a passive matrix type and an active matrix type. The passive matrix type light emitting device is provided with stripe shaped anode and cathode which are orthogonal to each other, and uses an EL element having a structure sandwiching an EL film between the anode and the cathode. Further, the active matrix type light emitting device is provided with a thin film transistor (hereinafter, referred to as a TFT) for each pixel, and is of a method for controlling current flowing in the EL element by the TFT connected to one of an anode and a cathode of an EL element.
Further, there are proposed various methods for color display of a light emitting device using EL elements. There is known a method for performing color display by mixing luminescence of colors, with three pixels, a pixel which emits red light, a pixel which emits green light and a pixel which emits blue light as one unit.
Such a method is attracting attention since a bright color display is easily obtained. However, since the EL elements which emit light of respective colors use different organic EL films as light emitting layers, the brightness characteristic of the light emitting layer (the relationship between operation voltage and the brightness) differs. As a result, the operation voltage necessary in obtaining a desired brightness differs for each EL element, and further the reliability (life) of the light emitting layer differs for each EL element.
From the above, it is feared that not only there will be an increase in the kinds of power sources necessary for the light emitting device, but also that a shift in the color balance due to difference in the life (deterioration rate) of the EL element will be generated.